The present invention relates to an apparatus with parallel plate electrodes for analyzing dielectric characteristics of a material as a function of temperature, frequency, or time.
Various techniques have been proposed for measuring the dielectric characteristic of a material as a function of temperature, frequency, or time and for evaluating the material on the basis of an electrical, chemical, or physical characteristic. The measurement generally is performed by arranging a sample between a pair of parallel plate electrodes and measuring the capacitance produced between the electrodes. The dielectric constant is obtained by applying the capacitance value to the following expression: EQU C=.epsilon..sub.r .epsilon..sub.0 (S/d)
where
C: Capacitance PA1 S: Electrode area PA1 d: Distance between electrodes (sample thickness) PA1 .epsilon..sub.r : Relative dielectric constant of a sample PA1 .epsilon..sub.0 : Dielectric constant of vacuum PA1 1/C.sub.M *=1/(C*+C.sub.R *)+2/C.sub.L * Expression (1) PA1 C*: Complex capacitance of a sample PA1 C.sub.R *: Complex capacitance of an insulating ring PA1 C.sub.L *: Complex capacitance of an insulating thin film PA1 C.sub.M *: Measured complex capacitance of the sandwiched sample. PA1 C*=(S.sub.R /t.sub.R).epsilon..sub.0 .epsilon.* Expression (2) PA1 C.sub.R *=((S-S.sub.R)/t.sub.R)).epsilon..sub.0 .epsilon..sub.R * Expression (3) PA1 C.sub.L *=(S/t.sub.L).epsilon..sub.0 .epsilon..sub.L * Expression (4) PA1 C.sub.M *=(S/t.sub.M).epsilon..sub.0 .epsilon..sub.M * Expression (5) PA1 t.sub.R : Thickness of insulating ring PA1 t.sub.L : Thickness of insulating thin film PA1 t.sub.M Thickness of a sandwiched sample (=t.sub.R +2t.sub.L) PA1 S: Electrode area PA1 S.sub.R : Inner area of an insulating ring PA1 .epsilon..sub.0 : Dielectric constant of vacuum PA1 .epsilon.*: Complex relative dielectric constant of a sample (=.epsilon.'-i.epsilon.") PA1 .epsilon.': Complex dielectric constant real part (storage dielectric constant) PA1 .epsilon.": Complex dielectric constant imaginary part (loss dielectric constant) PA1 .epsilon..sub.R *: Complex relative dielectric constant of an insulating ring (=.epsilon..sub.R '-i.epsilon..sub.R ") PA1 .epsilon..sub.L *: Complex relative dielectric constant of an insulating thin film (=.epsilon..sub.L'-i.epsilon..sub.L ") PA1 .epsilon..sub.M *: Measured complex relative dielectric constant of a sandwiched sample (=.epsilon..sub.M -i.epsilon..sub.M ")
Thus the dielectric constant of a sample can be measured by using an electrode with a known area and measuring the distance between the electrodes or the thickness of a certain sample.
However, measuring dielectric constants at varied temperatures can cause melting or softening of a sample, a change in the thickness of a sample upon cooling, and welding between an electrode and a sample, thus resulting in lowered measuring accuracy as well as experimental inconveniences. Effective countermeasures against those problems have been proposed as follows:
(1) An interdigital exciting electrode and a response electrode are arranged on a ceramic substrate so as to be close to each other. One example of such device is Micromet, Umetric, System II Option S-60 type ceramic sensor manufactured by Micromet Co. in U.S.A.);
(2) A sample sandwiched between insulating thin film members is measured using a parallel plate electrode; and
(3) A parallel plate electrode formed on a ceramic substrate is disposable if it experiences welding. The distance between the electrodes is measured and controlled. Such an arrangement is disclosed in Japanese Patent Application Laid-open No. 85770-1990.
The above proposed methods have the following disadvantages:
Method (1) cannot provide a spatially uniform electric field, comparing with the method using the parallel plate electrode. Since a sample is arranged in an uneven electric field, the measuring values depend disadvantageously on the shape of the sample. Furthermore since the electrode is formed on a single ceramic substrate, the measuring result involves a dielectric constant component of the ceramic substrate material, thus causing lowered accuracy of measurement.
Method (2) can prevent the contamination of an electrode due to welding because a sample in a fluid state touches directly to the electrode. However it is difficult to maintain constant the distance between the parallel plate electrodes during measurement. Particularly since the structure used in method (2) cannot prevent a sample from spreading in the radial direction, it is difficult to maintain a sample in a fixed shape during measurement. Furthermore the dielectric constant of the insulating material inserted between a sample and an electrode may cause a measuring error in the measured dielectric constant of the sample.
In method (3), the flatness of the electrode of a ceramic substrate tends to be degraded in comparison with that of an electrode made through conventional metal processing. In addition, when the ambient temperature of the electrodes is varied over several hundreds C.degree., it is extremely difficult to measure the distance between the electrodes to an accuracy of several .mu.m. Hence there is a disadvantage in that the method is impractical.